Nanotechnology boosts thermoelectric effect

PORTLAND, Ore. -- Thermoelectric coolers and power generators were handed a 40-percent boost in performance recently by a nanotechnological reconstruction of a classic bulk material. The technique is suitable for mass production, according to its inventors at Boston College and the Massachusetts Institute of Technology (MIT). This makes it of use in both industrial and consumer cooling applications from semiconductors to nanoscale power generators.

"Thermoelectric materials are already used in many applications," explains MIT professor Gang Chen, "[b]ut our more efficient nanoscale material will give a big boost to their performance."

The material improved by the researchers was bismuth antimony telluride, a common bulk semiconductor that has been used since the 1950s. It is used in a variety of applications from generating power for remote spacecraft at the National Aeronautics and Space Administration (NASA) to cooling automobile seats during summer heat waves. The auto industry has also been experimenting with ways of converting the heat from can exhausts into electric current that charges the batteries of hybrid cars.

Now all these applications, and even more, could be boosted by a process in which bulk bismuth antimony telluride is pulverized into nanoparticles as small as five nanometers before being reconstituted into a bulk material, bringing with it a 40-percent efficiency gain.

"Our technique is a low-cost method that can be easily scaled for mass production," said Boston College professor Zhifeng Ren. "Nanotechnology has enabled us to improve an old material by breaking it up and then rebuilding it into a composite of nanostructures in bulk form."

For cooling purposes, one end of a strip of bismuth antimony telluride is located inside the case of hot equipment and the other end outside. When a current is run through the material, heat is transferred from the inside end to the outside end, thus cooling the equipment without requiring fans.

Likewise, bismuth antimony telluride can work inversely when heat is applied to one end, causing a current to flow to the cool end, thus converting heat energy into electricity, power-generator style.

Unfortunately, bismuth antimony telluride also has a relatively high degree of thermal conductivity as a bulk material in addition to its thermoelectric properties, which causes parasitic loss. By pulverizing the material into tiny pieces and reconstituting it as a nanoscale bulk material, the researchers were able to lower its natural thermal conductivity, thus mitigating heat loss and achieving a 40-percent gain in efficiency.

Because bismuth antimony telluride is an insulating solid, quantum vibrations called phonons are thought to be the primary mechanism by which heat is parasitically lost in the material. The pulverizing and reconstitution steps, however, create a nanoscale patchwork of grains and irregularities that inhibits the passage of phonons through the material and reduces parasitic heat loss, boosting the material's thermoelectric performance. Characterization of the new nanoscale material revealed that it retains its higher efficiency over a wide range of temperatures: from room temperature up to 482 degrees Fahrenheit.

Also contributing to the work were MIT Professor Mildred Dresselhaus, Boston College research scientist Bed Poudel and CEO Mike Clary of GMZ Energy, Inc. (Newton, Mass.), as well as Boston College visiting professor Junming Liu and a visiting physicist from Nanjing University (China). The research was funded by the Department of Energy and the National Science Foundation.